In a field of vehicle such as an automobile and the like, a steering angle control device such as a steering transmission ratio varying unit and a steerable wheel steering device of steer-by-wire type is well known which alters the relationship between the amount of steering operation of a driver and the steering angle of steerable wheels. Various travel control devices for vehicles have been proposed which execute vehicle stability control for enhancing stability of vehicle turn behavior in vehicles provided with such steering angle control devices.
For example, in Japanese Patent Laid-Open Publication No. 2006-117070 is described a travel control device for a vehicle which controls the steering angle of steerable wheels by means of a steering angle control device so that the vehicle travels along a target travel line and, when the steering angle of steerable wheels is controlled by means of a steering angle control device, lowers the start condition level of a vehicle stability control as compared with the case where the steering angle of steerable wheels is not controlled.
According to the above-described conventional travel control device, the start condition level of the vehicle stability control is lowered when the turning behavior stability of the vehicle decreases under the situation where a vehicle travel control is executed which controls the steering angle of steerable wheels by means of a steering angle control device. Accordingly, when a vehicle travel control is executed, vehicle stability control can be rendered to be started earlier, which enables to stabilize the turn behavior stability of the vehicle earlier.
In such a conventional travel control device as described in the above publication, however, when a travel trajectory control is executed, the steering angle control device alters the relationship between the amount of steering operation of a driver and the steering angle of steerable wheels, and, accordingly, the amount of steering operation of a driver does not correspond to the actual steering angle of the steerable wheels any more.
In spite of the above, in such a conventional travel control device as described in the above publication, a target turning state quantity of a vehicle is calculated on the basis of the amount of steering operation of a driver and turning state quantity of the vehicle is controlled to be the target turn state quantity of the vehicle. Accordingly, a target control level of the vehicle stability control can not be calculated to be a suitable value which reflects the change of the steering angle of steerable wheels caused by the travel trajectory control and, due to this, the vehicle stability control can not be properly executed.
To cope with the above problem, it may be considered to calculate a target turn state quantity of a vehicle on the basis of actual steering angle of steerable wheels. However, in the case where the vehicle stability control is executed by means of the control of braking-driving force of each wheel and the control of steering angle of steerable wheels, actual steering angle of steerable wheels becomes a value which reflects the both control results of the travel trajectory control and the vehicle stability control. Accordingly, the turn state quantity of the vehicle which is calculated on the basis of actual steering angle of steerable wheels includes error components corresponding to the control level of steering angle of steerable wheels by the vehicle stability control and, due to this, the vehicle stability control can not be properly executed as well.
Furthermore, the above-mentioned two problems are not inherent in the case where the control of steering angle of steerable wheels is conducted by a travel trajectory control, but arise in the cases where a steering characteristic control for altering a steering transmission ratio is executed or both a travel trajectory control and a steering characteristic control are executed.